How do u value your games to trade?

The best way to find a price is just look around. Basic supply and demand applies here as well (a lot of demand but low supply means higher prices, a lot of supply but no demand means low prices). Another good indicator is the price on the marketplaces.

I personally don't trade much, but I trade only for games I'm interested in that seem to be around the same value.

Depends on the quality and source, but you can use these things to get an idea:
G2a, how much you paid for it,lowest price ever, ratings, if it has cards, availability, if it's a code or a gift, etc.

If game was given for free - number of card matters (plus something extra if it was not in given in mass quantities, and just once, long time ago).

Otherwise market decides - number of times it was bundled, how big was max discount, was it bundled recently, cards status, ratings, and finally retail price (as nearly last factor).

It also depends what you're trading, the situation looks a bit different with bundled keys vs gifts.

It depends on the lowest price it has been sold in the past, the prices that they're been selling it around, if you have monopoly of this game, the available copies on the market, the price that you bought it yourself, how trustworthy you are (because people prefer people with a lot of +rep, even with tradable copies), etc. xD Yeah, I know, it's a bit hard to calculate it.

always treat your games worth 20 keys and everyone else's worth 4 cards. :3

first i look if the game was bundled or not.
then the price. if bundled, its worth very little unless its BTA. if not, the price in keys is equal or slightly more/less than sale price.
that is how i decide on price. although im not exactly a hardcore trader...

If a game was free its value is a couple cards at best